[PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
was using an alignment of 4 bytes (sizeof(u32)), which could lead
to the 'target' field being misaligned in the JIT buffer.

Increase the alignment requirement to 8 bytes (sizeof(u64)) in
bpf_jit_binary_pack_alloc() to guarantee proper alignment for
struct bpf_plt.

Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/net/bpf_jit_comp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 356d33c7a4ae..adf84962d579 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
 	image_size = extable_offset + extable_size;
 	ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
-					      sizeof(u32), &header, &image_ptr,
+					      sizeof(u64), &header, &image_ptr,
 					      jit_fill_hole);
 	if (!ro_header) {
 		prog = orig_prog;
-- 
2.53.0.371.g1d285c8824-goog

[PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
was using an alignment of 4 bytes (sizeof(u32)), which could lead
to the 'target' field being misaligned in the JIT buffer.

Increase the alignment requirement to 8 bytes (sizeof(u64)) in
bpf_jit_binary_pack_alloc() to guarantee proper alignment for
struct bpf_plt.

Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/net/bpf_jit_comp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 356d33c7a4ae..adf84962d579 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
 	image_size = extable_offset + extable_size;
 	ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
-					      sizeof(u32), &header, &image_ptr,
+					      sizeof(u64), &header, &image_ptr,
 					      jit_fill_hole);
 	if (!ro_header) {
 		prog = orig_prog;
-- 
2.53.0.371.g1d285c8824-goog

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Will Deacon]

On Tue, Feb 24, 2026 at 09:29:15AM +0000, Fuad Tabba wrote:
> struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> was using an alignment of 4 bytes (sizeof(u32)), which could lead
> to the 'target' field being misaligned in the JIT buffer.
> 
> Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> struct bpf_plt.
> 
> Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/net/bpf_jit_comp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 356d33c7a4ae..adf84962d579 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  	extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
>  	image_size = extable_offset + extable_size;
>  	ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> -					      sizeof(u32), &header, &image_ptr,
> +					      sizeof(u64), &header, &image_ptr,
>  					      jit_fill_hole);

Did you see a functional issue with this or are you just trying to squash
a UBSAN splat? I can't see an issue with the code and it seems a bit
over-the-top to over-align the whole JIT buffer just because of structure
alignment rules that don't really make sense for the actual PLT.

Does marking 'struct bpf_plt' as __packed help?

Will

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Xu Kuohai]

On 2/24/2026 5:31 PM, Fuad Tabba wrote:
> struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> was using an alignment of 4 bytes (sizeof(u32)), which could lead
> to the 'target' field being misaligned in the JIT buffer.
> 
> Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> struct bpf_plt.
> 
> Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   arch/arm64/net/bpf_jit_comp.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 356d33c7a4ae..adf84962d579 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>   	extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
>   	image_size = extable_offset + extable_size;
>   	ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> -					      sizeof(u32), &header, &image_ptr,
> +					      sizeof(u64), &header, &image_ptr,
>   					      jit_fill_hole);
>   	if (!ro_header) {
>   		prog = orig_prog;

Good catch. build_plt pads NOP instructions to ensure a 64-bit relative offset for
plt target, but it misses the alignment check for image base itself.

Acked-by: Xu Kuohai <xukuohai@huaweicloud.com>

nit: Add check for base alignment in build_plt, or a comment to clarify?

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

Hi Will,

On Tue, 24 Feb 2026 at 22:11, Will Deacon <will@kernel.org> wrote:
>
> On Tue, Feb 24, 2026 at 09:29:15AM +0000, Fuad Tabba wrote:
> > struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> > was using an alignment of 4 bytes (sizeof(u32)), which could lead
> > to the 'target' field being misaligned in the JIT buffer.
> >
> > Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> > bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> > struct bpf_plt.
> >
> > Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >  arch/arm64/net/bpf_jit_comp.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > index 356d33c7a4ae..adf84962d579 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >       extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
> >       image_size = extable_offset + extable_size;
> >       ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> > -                                           sizeof(u32), &header, &image_ptr,
> > +                                           sizeof(u64), &header, &image_ptr,
> >                                             jit_fill_hole);
>
> Did you see a functional issue with this or are you just trying to squash
> a UBSAN splat? I can't see an issue with the code and it seems a bit
> over-the-top to over-align the whole JIT buffer just because of structure
> alignment rules that don't really make sense for the actual PLT.

I didn't see a functional issue. I was debugging something else, and
ran into the UBSAN splat.

> Does marking 'struct bpf_plt' as __packed help?

It does. It drops the compiler's alignment assumption silencing the
UBSAN warning without touching the allocator, and the generated
assembly is identical. I'll respin a v2 with that fix instead.

Thanks,
/fuad

>
> Will

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

Hi Xu,

On Wed, 25 Feb 2026 at 01:43, Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>
> On 2/24/2026 5:31 PM, Fuad Tabba wrote:
> > struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> > was using an alignment of 4 bytes (sizeof(u32)), which could lead
> > to the 'target' field being misaligned in the JIT buffer.
> >
> > Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> > bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> > struct bpf_plt.
> >
> > Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >   arch/arm64/net/bpf_jit_comp.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > index 356d33c7a4ae..adf84962d579 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >       extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
> >       image_size = extable_offset + extable_size;
> >       ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> > -                                           sizeof(u32), &header, &image_ptr,
> > +                                           sizeof(u64), &header, &image_ptr,
> >                                             jit_fill_hole);
> >       if (!ro_header) {
> >               prog = orig_prog;
>
> Good catch. build_plt pads NOP instructions to ensure a 64-bit relative offset for
> plt target, but it misses the alignment check for image base itself.
>
> Acked-by: Xu Kuohai <xukuohai@huaweicloud.com>
>
> nit: Add check for base alignment in build_plt, or a comment to clarify?

Thanks for the having a look and for the Ack.

You're right that build_plt() assumes 64-bit alignment when
calculating the NOP padding. However, Will pointed out, over-aligning
the entire JIT buffer just to satisfy the C standard is somewhat
heavy-handed. I didn't actually run into a functional bug. The issue
is that UBSAN complains because we violate the standard's alignment
rules.
I'll dropping the allocator change in favor of marking struct bpf_plt
as __packed.

Thanks again,
/fiad

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Xu Kuohai]

On 2/25/2026 5:08 PM, Fuad Tabba wrote:
> Hi Xu,
> 
> On Wed, 25 Feb 2026 at 01:43, Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>>
>> On 2/24/2026 5:31 PM, Fuad Tabba wrote:
>>> struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
>>> was using an alignment of 4 bytes (sizeof(u32)), which could lead
>>> to the 'target' field being misaligned in the JIT buffer.
>>>
>>> Increase the alignment requirement to 8 bytes (sizeof(u64)) in
>>> bpf_jit_binary_pack_alloc() to guarantee proper alignment for
>>> struct bpf_plt.
>>>
>>> Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
>>> Signed-off-by: Fuad Tabba <tabba@google.com>
>>> ---
>>>    arch/arm64/net/bpf_jit_comp.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>>> index 356d33c7a4ae..adf84962d579 100644
>>> --- a/arch/arm64/net/bpf_jit_comp.c
>>> +++ b/arch/arm64/net/bpf_jit_comp.c
>>> @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>>        extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
>>>        image_size = extable_offset + extable_size;
>>>        ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
>>> -                                           sizeof(u32), &header, &image_ptr,
>>> +                                           sizeof(u64), &header, &image_ptr,
>>>                                              jit_fill_hole);
>>>        if (!ro_header) {
>>>                prog = orig_prog;
>>
>> Good catch. build_plt pads NOP instructions to ensure a 64-bit relative offset for
>> plt target, but it misses the alignment check for image base itself.
>>
>> Acked-by: Xu Kuohai <xukuohai@huaweicloud.com>
>>
>> nit: Add check for base alignment in build_plt, or a comment to clarify?
> 
> Thanks for the having a look and for the Ack.
> 
> You're right that build_plt() assumes 64-bit alignment when
> calculating the NOP padding. However, Will pointed out, over-aligning
> the entire JIT buffer just to satisfy the C standard is somewhat
> heavy-handed. I didn't actually run into a functional bug. The issue
> is that UBSAN complains because we violate the standard's alignment
> rules.
> I'll dropping the allocator change in favor of marking struct bpf_plt
> as __packed.
>

Interesting, I think the plt target should be 64-bit aligned to ensure
atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
in the bpf_arch_text_poke function while the ldr instruction in the plt is
executed. If it is not aligned correctly, the ldr may read a half-old
half-new value, causing the plt to jump to an invalid destination.

To avoid over-aligning the entire buffer, how about fixing the padding
method in build_plt to just make the plt target aligned correctly?


> Thanks again,
> /fiad

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

Hi Xu,

On Wed, 25 Feb 2026 at 09:46, Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>
> On 2/25/2026 5:08 PM, Fuad Tabba wrote:
> > Hi Xu,
> >
> > On Wed, 25 Feb 2026 at 01:43, Xu Kuohai <xukuohai@huaweicloud.com> wrote:
> >>
> >> On 2/24/2026 5:31 PM, Fuad Tabba wrote:
> >>> struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> >>> was using an alignment of 4 bytes (sizeof(u32)), which could lead
> >>> to the 'target' field being misaligned in the JIT buffer.
> >>>
> >>> Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> >>> bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> >>> struct bpf_plt.
> >>>
> >>> Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> >>> Signed-off-by: Fuad Tabba <tabba@google.com>
> >>> ---
> >>>    arch/arm64/net/bpf_jit_comp.c | 2 +-
> >>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> >>> index 356d33c7a4ae..adf84962d579 100644
> >>> --- a/arch/arm64/net/bpf_jit_comp.c
> >>> +++ b/arch/arm64/net/bpf_jit_comp.c
> >>> @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >>>        extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
> >>>        image_size = extable_offset + extable_size;
> >>>        ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> >>> -                                           sizeof(u32), &header, &image_ptr,
> >>> +                                           sizeof(u64), &header, &image_ptr,
> >>>                                              jit_fill_hole);
> >>>        if (!ro_header) {
> >>>                prog = orig_prog;
> >>
> >> Good catch. build_plt pads NOP instructions to ensure a 64-bit relative offset for
> >> plt target, but it misses the alignment check for image base itself.
> >>
> >> Acked-by: Xu Kuohai <xukuohai@huaweicloud.com>
> >>
> >> nit: Add check for base alignment in build_plt, or a comment to clarify?
> >
> > Thanks for the having a look and for the Ack.
> >
> > You're right that build_plt() assumes 64-bit alignment when
> > calculating the NOP padding. However, Will pointed out, over-aligning
> > the entire JIT buffer just to satisfy the C standard is somewhat
> > heavy-handed. I didn't actually run into a functional bug. The issue
> > is that UBSAN complains because we violate the standard's alignment
> > rules.
> > I'll dropping the allocator change in favor of marking struct bpf_plt
> > as __packed.
> >
>
> Interesting, I think the plt target should be 64-bit aligned to ensure
> atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
> in the bpf_arch_text_poke function while the ldr instruction in the plt is
> executed. If it is not aligned correctly, the ldr may read a half-old
> half-new value, causing the plt to jump to an invalid destination.

You're right. I missed that target is concurrently updated via
bpf_arch_text_poke() and read by ldr. If target crosses an 8-byte
boundary, we lose the single-copy atomicity guarantee, risking a torn
read. So I guess that this isn't just UBSAN, it could cause real
issues...

> To avoid over-aligning the entire buffer, how about fixing the padding
> method in build_plt to just make the plt target aligned correctly?

I'm not sure about this. If my reading of the code is correct, during
the first JIT pass, ctx->image is NULL. The current padding logic in
build_plt() looks like this:

        /* make sure target is 64-bit aligned */
        if ((ctx->idx + PLT_TARGET_OFFSET / AARCH64_INSN_SIZE) % 2)
            emit(A64_NOP, ctx);

This forces the relative offset of the PLT to be a multiple of 8
bytes. Therefore, it assumes that the base pointer (ctx->image) is
also 8-byte aligned. If the allocator gives us a base pointer that is
only 4-byte aligned, target will end up misaligned.

If we try to make the padding dynamic based on the actual address of
ctx->image, pass 1 (where ctx->image is NULL) and pass 2 (where
ctx->image is allocated) might disagree on the number of NOPs
required. This would cause ctx->idx to diverge between passes,
breaking the size calculations and offset tables.

Given that enforcing 8-byte alignment in bpf_jit_binary_pack_alloc
wastes a maximum of 4 bytes per BPF program, I think my original v1
patch was actually the safest and cleanest way to fix both the UBSAN
warning and the tearing risk.

What do you think? If you agree, I will abandon v2, and resubmit the
original one as v3 with an updated commit message detailing your
observation regarding the atomic read/write requirement.

Thanks,
/fuad

>
> > Thanks again,
> > /fiad
>

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Will Deacon]

On Wed, Feb 25, 2026 at 05:46:52PM +0800, Xu Kuohai wrote:
> Interesting, I think the plt target should be 64-bit aligned to ensure
> atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
> in the bpf_arch_text_poke function while the ldr instruction in the plt is
> executed. If it is not aligned correctly, the ldr may read a half-old
> half-new value, causing the plt to jump to an invalid destination.

Thanks for pointing that out; I hadn't realised that we patched live
PLTs!

> To avoid over-aligning the entire buffer, how about fixing the padding
> method in build_plt to just make the plt target aligned correctly?

Makes sense to me.

Will

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

Hi Will,

On Wed, 25 Feb 2026 at 17:48, Will Deacon <will@kernel.org> wrote:
>
> On Wed, Feb 25, 2026 at 05:46:52PM +0800, Xu Kuohai wrote:
> > Interesting, I think the plt target should be 64-bit aligned to ensure
> > atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
> > in the bpf_arch_text_poke function while the ldr instruction in the plt is
> > executed. If it is not aligned correctly, the ldr may read a half-old
> > half-new value, causing the plt to jump to an invalid destination.
>
> Thanks for pointing that out; I hadn't realised that we patched live
> PLTs!
>
> > To avoid over-aligning the entire buffer, how about fixing the padding
> > method in build_plt to just make the plt target aligned correctly?
>
> Makes sense to me.

As I noted in my reply to Xu, and keeping in mind that this is very
new to me (so take it with a huge pinch of salt), I don't think this
is correct, but that my original patch is probably the best fix:

> I'm not sure about this. If my reading of the code is correct, during
> the first JIT pass, ctx->image is NULL. The current padding logic in
> build_plt() looks like this:
>
>         /* make sure target is 64-bit aligned */
>         if ((ctx->idx + PLT_TARGET_OFFSET / AARCH64_INSN_SIZE) % 2)
>             emit(A64_NOP, ctx);
>
> This forces the relative offset of the PLT to be a multiple of 8
> bytes. Therefore, it assumes that the base pointer (ctx->image) is
> also 8-byte aligned. If the allocator gives us a base pointer that is
> only 4-byte aligned, target will end up misaligned.
>
> If we try to make the padding dynamic based on the actual address of
> ctx->image, pass 1 (where ctx->image is NULL) and pass 2 (where
> ctx->image is allocated) might disagree on the number of NOPs
> required. This would cause ctx->idx to diverge between passes,
> breaking the size calculations and offset tables.

Cheers,
/fuad

>
> Will

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Will Deacon]

On Wed, Feb 25, 2026 at 05:53:18PM +0000, Fuad Tabba wrote:
> On Wed, 25 Feb 2026 at 17:48, Will Deacon <will@kernel.org> wrote:
> >
> > On Wed, Feb 25, 2026 at 05:46:52PM +0800, Xu Kuohai wrote:
> > > Interesting, I think the plt target should be 64-bit aligned to ensure
> > > atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
> > > in the bpf_arch_text_poke function while the ldr instruction in the plt is
> > > executed. If it is not aligned correctly, the ldr may read a half-old
> > > half-new value, causing the plt to jump to an invalid destination.
> >
> > Thanks for pointing that out; I hadn't realised that we patched live
> > PLTs!
> >
> > > To avoid over-aligning the entire buffer, how about fixing the padding
> > > method in build_plt to just make the plt target aligned correctly?
> >
> > Makes sense to me.
> 
> As I noted in my reply to Xu, and keeping in mind that this is very
> new to me (so take it with a huge pinch of salt), I don't think this
> is correct, but that my original patch is probably the best fix:

[...]

> > I'm not sure about this. If my reading of the code is correct, during
> > the first JIT pass, ctx->image is NULL. The current padding logic in
> > build_plt() looks like this:
> >
> >         /* make sure target is 64-bit aligned */
> >         if ((ctx->idx + PLT_TARGET_OFFSET / AARCH64_INSN_SIZE) % 2)
> >             emit(A64_NOP, ctx);

Aha, so this handles the case where we have an odd number of instructions
and need to chuck out a NOP to align the PLT target. So, yes, I agree
that aligning the entire region to 8 bytes seems to be the expected
behaviour, otherwise that NOP could actually result in a misaligned
address!

Will

Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Xu Kuohai]

On 2/25/2026 7:00 PM, Fuad Tabba wrote:
> Hi Xu,
> 
> On Wed, 25 Feb 2026 at 09:46, Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>>
>> On 2/25/2026 5:08 PM, Fuad Tabba wrote:
>>> Hi Xu,
>>>
>>> On Wed, 25 Feb 2026 at 01:43, Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>>>>
>>>> On 2/24/2026 5:31 PM, Fuad Tabba wrote:
>>>>> struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
>>>>> was using an alignment of 4 bytes (sizeof(u32)), which could lead
>>>>> to the 'target' field being misaligned in the JIT buffer.
>>>>>
>>>>> Increase the alignment requirement to 8 bytes (sizeof(u64)) in
>>>>> bpf_jit_binary_pack_alloc() to guarantee proper alignment for
>>>>> struct bpf_plt.
>>>>>
>>>>> Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
>>>>> Signed-off-by: Fuad Tabba <tabba@google.com>
>>>>> ---
>>>>>     arch/arm64/net/bpf_jit_comp.c | 2 +-
>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>>>>> index 356d33c7a4ae..adf84962d579 100644
>>>>> --- a/arch/arm64/net/bpf_jit_comp.c
>>>>> +++ b/arch/arm64/net/bpf_jit_comp.c
>>>>> @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>>>>         extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
>>>>>         image_size = extable_offset + extable_size;
>>>>>         ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
>>>>> -                                           sizeof(u32), &header, &image_ptr,
>>>>> +                                           sizeof(u64), &header, &image_ptr,
>>>>>                                               jit_fill_hole);
>>>>>         if (!ro_header) {
>>>>>                 prog = orig_prog;
>>>>
>>>> Good catch. build_plt pads NOP instructions to ensure a 64-bit relative offset for
>>>> plt target, but it misses the alignment check for image base itself.
>>>>
>>>> Acked-by: Xu Kuohai <xukuohai@huaweicloud.com>
>>>>
>>>> nit: Add check for base alignment in build_plt, or a comment to clarify?
>>>
>>> Thanks for the having a look and for the Ack.
>>>
>>> You're right that build_plt() assumes 64-bit alignment when
>>> calculating the NOP padding. However, Will pointed out, over-aligning
>>> the entire JIT buffer just to satisfy the C standard is somewhat
>>> heavy-handed. I didn't actually run into a functional bug. The issue
>>> is that UBSAN complains because we violate the standard's alignment
>>> rules.
>>> I'll dropping the allocator change in favor of marking struct bpf_plt
>>> as __packed.
>>>
>>
>> Interesting, I think the plt target should be 64-bit aligned to ensure
>> atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
>> in the bpf_arch_text_poke function while the ldr instruction in the plt is
>> executed. If it is not aligned correctly, the ldr may read a half-old
>> half-new value, causing the plt to jump to an invalid destination.
> 
> You're right. I missed that target is concurrently updated via
> bpf_arch_text_poke() and read by ldr. If target crosses an 8-byte
> boundary, we lose the single-copy atomicity guarantee, risking a torn
> read. So I guess that this isn't just UBSAN, it could cause real
> issues...
> 
>> To avoid over-aligning the entire buffer, how about fixing the padding
>> method in build_plt to just make the plt target aligned correctly?
> 
> I'm not sure about this. If my reading of the code is correct, during
> the first JIT pass, ctx->image is NULL. The current padding logic in
> build_plt() looks like this:
> 
>          /* make sure target is 64-bit aligned */
>          if ((ctx->idx + PLT_TARGET_OFFSET / AARCH64_INSN_SIZE) % 2)
>              emit(A64_NOP, ctx);
> 
> This forces the relative offset of the PLT to be a multiple of 8
> bytes. Therefore, it assumes that the base pointer (ctx->image) is
> also 8-byte aligned. If the allocator gives us a base pointer that is
> only 4-byte aligned, target will end up misaligned.

> If we try to make the padding dynamic based on the actual address of
> ctx->image, pass 1 (where ctx->image is NULL) and pass 2 (where
> ctx->image is allocated) might disagree on the number of NOPs
> required. This would cause ctx->idx to diverge between passes,
> breaking the size calculations and offset tables.
>

Right, I missed it causing ctx->idx to diverge. Thanks for the
explanation!

> Given that enforcing 8-byte alignment in bpf_jit_binary_pack_alloc
> wastes a maximum of 4 bytes per BPF program, I think my original v1
> patch was actually the safest and cleanest way to fix both the UBSAN
> warning and the tearing risk.
> 
> What do you think? If you agree, I will abandon v2, and resubmit the
> original one as v3 with an updated commit message detailing your
> observation regarding the atomic read/write requirement.
>

Makes sense to me.

> Thanks,
> /fuad
> 
>>
>>> Thanks again,
>>> /fiad
>>